The invention relates to a process for the continuous production of boards of wood-based material, primarily chipboards/fiberboards. The invention further relates to a system for carrying out the process according to the invention.
A process is known from DE-B-24 25 638. According to the latter, the intention is to increase the output of finished boards, in particular in the production of chipboards or fiberboards, by a continuously operating press or to reduce the length of the continuously operating press. For this purpose, after a preliminary pressing operation, the pressed material mat is preheated. It has been found to be a disadvantage of this that, after leaving the continuously operating main press, the finished boards still have troublesome steam occlusions, which can be prevented only by expensive and complicated steam removal measures within the pressing zone of the continuously operating main press.
The use of hot gases, primarily highly pressurized steam, in the continuously operating presses is disclosed by DE-A-20 58 820. The superheated steam is fed to the pressed material through the gas-permeable belts (meshed metal belts, or steel belts perforated in the manner of a screen). These gas-permeable steel belts are supported by means of rolling supporting elements, which in turn are supported against a rolling surface. In principle, the feeding of the hot gas takes place within the main press, the feeding of the gas along the pressing zone taking place in terms of rate and temperature within the curing and calibrating zone, that is to say in the high-compaction region of the continuously operating main press. The disadvantage of this apparatus for carrying out the process is that, due to the rolling supporting elements, the superheated steam flows through a relatively long steam-depressurization zone in an uncontrolled manner, and consequently cools to varying degrees. Secondly, the gas-permeable steel belt structure on the surfaces of the chipboards or fiberboards produced gives rise to a marked surface texture, which has to be subsequently ground off, additional production costs arising due to material losses.
A further configuration of the preheating of the pressed material mats by a steam jet is represented by the apparatus published in German Patent 39 14 106, with the advantage that, at least in the main press, nonperforated, smooth steel belts are used, so that no surface markings occur on the finished pressed boards. A novel feature of this patent is the rule for proportioning the residual moisture of the pressed material mat after leaving the steam treatment zone before entry into the pressing nip of the continuously operating main press. This residual moisture, that is the controlled moisture, of preferably 12 percent by weight is set from a pressed material mat whose initial moisture is less than this known controlled moisture, because the water content of the pressed material mat is raised again by the feeding of steam, it being intended for the pressed material mat to be raised in temperature by about 60.degree. Celsius, that is to say to a maximum of 80.degree. C. It has been found to be a disadvantage that the water content of the pressed material mat, increased by the feeding of steam, has to be extracted again, that is to say, contrary to the aim stated as the object, a not inconsiderable part of the pressing zone of the main press is again required for evaporating the moisture introduced, because the finished pressed board is intended, according to practice thus far in technological applications, to leave the continuously operating main press with a residual moisture of about six percent by weight. On account of the increased temperature of the pressed material mat up to 80.degree. Celsius, the controlled moisture of on average 12 percent by weight is no longer required in the full amount as a "means of heat conduction". Furthermore, due to the high entry temperature of about 80.degree. Celsius and the high controlled moisture of about 12.5 percent by weight, the pressed material mat becomes so highly plastic that a higher compaction in the outer layer regions of the boards is possible only to a limited extent. This disadvantage also applies in the same way to the industrial property rights already mentioned above and to the processes described above. Due to the inadequate compaction of the outer layer regions, the usefulness of the boards thus produced is also considerably impaired. That it to say that the boards not only have a lower bending resistance but also an inadequate surface hardness and, as a result, are much less suitable for painting and/or laminating.
The present invention is based on the object of further developing the process of continuously producing boards of wood-based material, primarily chipboards/fiberboards, such that a preheating and precompaction of the pressed material mat is made possible without additional steam-removal measures being required within the main pressing region, it being intended for the preheating temperature to bring about shorter setting and crosslinking times within the continuously operating main press and leading as a result to a marked increase in production with shorter pressing times and also achieving a higher compaction of the outer layer regions in comparison with the middle core region in the finished pressed board. The invention is also based on the object of providing an installation which is particularly suitable for carrying out the developed process.
Also to be mentioned as an advantage of the present invention is that, up until entry into the main pressing region, or up until entry into the run-in nip of the continuously operating press, the pressed material mat is regulated during preheating to a temperature just a little below the initial polymerization of the binder and the moisture level is regulated below the controlled moisture, the initial polymerization temperature generally lying between 60.degree. Celsius and 100.degree. Celsius steam point, depending on the reaction time of the binder.